I've been playing lots of games online, but my personal favorites are collectible card games. Here are some of the games I've played:
Urban Rivals - this one's easy to understand.Some of the card art are fantastic too.
Kongai -hard in the beginning but it can get crazy once you get the best cards.
Eredan - never really liked this game. I think it's not very well-balanced.
Card Hunter - still undergoing development but THIS ONE... This one may just be the "Glitch" of online card games. Can't wait to play it!